In a drawing process of quartz glass, the outer surface of an optical fiber is immediately coated with coating resin to prevent the degradation in strength of the optical fiber. As the optical fiber coating resin, ultraviolet cured resin is mainly used. As the ultraviolet cured resin, a urethane acrylate based resin or epoxy acrylate based resin is used.
In an optical fiber, transmission loss increases due to various external stress and microbends thereby caused. Therefore, to protect the optical fiber from such external stress, an optical fiber is generally provided with a coating having a two-layer structure. Resin with relatively low Young's modulus is used for an inner layer which contacts quartz glass as a buffer layer (hereinafter referred to as a “primary layer”) and resin with relatively high Young's modulus is used for an outer layer as a protection layer (hereinafter referred to as a “secondary layer”).
The optical fiber is fabricated by producing a quartz glass optical fiber from melting a quartz glass based preform by heating and drawing, using a drawing furnace, then coating with a primary layer and a secondary layer by applying ultraviolet cured resin, using a coating die, to the quartz glass optical fiber, and irradiating this with ultraviolet rays to harden the ultraviolet cured resin. In the next step, the optical fiber produced in this way is coated with a colored layer for identification and a colored optical fiber as shown in FIG. 1 is obtained. An optical fiber coated with coating resin may be generally called an “optical fiber” and one further provided with a colored layer may be called an “colored optical fiber” but an optical fiber subjected to any coating will be referred to as an “optical fiber” in the present specification for convenience of explanation.
Ultraviolet cured resin is generally used for a colored layer, too. A plurality of colored optical fibers obtained as described above are then arranged in parallel as shown in FIG. 2, the outer surface thereof is coated with, for example, a ribbon layer made of ultraviolet cured resin all together, irradiated with ultraviolet rays to harden the ribbon layer to thereby use it as an optical fiber ribbon. Such an optical fiber ribbon is often used mainly as a high density optical fiber cable.
When an optical fiber is used immersed in water for a long period of time, transmission loss may increase. For example, Japanese Patent No. 2925099 (Patent Document 1) discloses that when an optical fiber ribbon is used in a high humidity atmosphere, extraordinary swelling occurs between a secondary layer and a colored layer or between a colored layer and a ribbon layer due to moisture absorption and this causes stress to apply to quartz glass and thereby causes transmission loss to increase. According to the above Patent Document 1, an increase in transmission loss is restrained by optimizing a weight change rate due to water immersion of ink as the colored layer.
With a significant widespread use of optical fibers in recent years, the application of optical fiber cables is expanding. This means that an environment in which optical fiber cables are used is more and more diversified. For this reason, the long-term reliability required for an optical fiber cable is becoming more severe.
Combined with such a situation, even when the weight change rate due to water immersion of ink as the colored layer is optimized as described in the above Patent Document 1, the optical fiber may still be exposed to water immersion and transmission loss may increase.
The inventors have made every effort to study the cause for this and have discovered that in an optical fiber exposed to water immersion and with increased transmission loss, peeling, that is, delamination is often observed not only in a secondary layer/colored layer interface or a colored layer/ribbon layer interface but also in a quartz optical fiber/primary layer interface. When delamination occurs in the quartz optical fiber/primary interface in this way, stress in the part becomes nonuniform and transmission loss increases in the optical fiber by microbends. An issue to be addressed in the present invention is to provide an optical fiber and an optical fiber ribbon whose transmission loss hardly increases even when the optical fiber accommodated in an optical fiber cable is affected in an operating environment and aged deterioration, especially when exposed to water.